Designing against cheating

Soon after the first version reached real users, a problem emerged that no one on the team had anticipated. Students discovered that they could lie on the floor, bend their arms slightly and collect points for push-ups without lifting their body at all. The feature that was meant to reward effort was rewarding the opposite.

The team rewrote the detector around three layers of checks. The first layer examines posture: the torso must be close to horizontal and the line from shoulder to hip to ankle must stay almost straight. The second layer measures real movement: the shoulder has to drop by at least forty per cent of the length of the upper arm, and the elbow must bend past one hundred degrees before straightening again. Because the drop is measured in units of the student's own arm, the rule is fair whether the camera is near or far. The third layer checks rhythm: the lowest position must be held briefly, each repetition needs at least six tenths of a second, and a long pause cancels the attempt.

Eleven automated scenarios now run against the detector, including lying still, bending only the arms, and flickering noise. Old scores were recalculated so that nobody kept an unfair advantage, and the history was preserved for inspection.